Seasons and Solstices

Why are there seasons? For many years, people pondered this same question, and it took humankind until the 1500s to finally figure it all out. The solution, it turns out, is as simple as the Earth orbiting the Sun. Almost every person born since the Civil War has known this since he (or she) was a child, but still, just because the Earth goes 'round the sun, this does not imply that seasons should necessarily occur; there's something a little more complex than just the orbit of the Earth involved. This section of the tutorial shall examine how it all works, and why, exactly, there are seasons.

A few terms you ought to know

Equinox Solstice

The Axis of the Earth

The Earth spins on its axis, an invisible line through the center of the Earth. The northernmost point of this axis is the North Pole. The southernmost point, therefore, is the South Pole. The Equator is an invisible line that encircles the widest point of the Earth, and is equidistant from either pole at every point; that is to say, the Equator is the same distance away from each pole at every point along it.

The Earth rotates along the plane of the equator, meaning that the Earth spins in a circle represented by the equator, or any latitude line, for that matter. (It wobbles a little, actually, but it's such a small wobble that it really doesn't matter much unless you're thinking in terms of 26,000 years at a time.) If you stood out in space so that you could look down over the North Pole, you would notice that the Earth spins counterclockwise, which makes sense if you consider that the sun rises in the East and sets in the West.

North Polar and Equatorial views of Earth and Solar Beams

The interesting thing about all this is that, even though the Earth rotates on the plane of the equator, the Sun doesn't always follow the equatorial path. In fact, the plane that the Sun appears to follow as Earth rotates is dependent upon the time of the year. It's a little tough to grasp at first, but it's this tilt that gives us the seasons.

Earth's axis tilt and equatorial plane compared with its plane of orbit

Even atmospheric scientists have trouble with this concept at first, so don't be discouraged. If it still isn't that clear, click on the picture, and you'll go to a place with instant, Java-friendly animations, and a more extensive explanation.